Developing one-dimensional implosions for inertial confinement fusion science Download: 818次
1 Introduction
While progress towards laser-based indirect drive inertial confinement fusion (ICF) is being made[1–3], experimental results show challenges remain. Recent modeling of implosion experiments for both low and high adiabat implosions has identified specific issues that are believed to impact the performance for each type of implosion[4]. For the high convergence, low adiabat implosions, the dominant mechanisms believed to degrade performance are hydrodynamic instabilities seeded by capsule mounting hardware such as the fill tube, tenting[5, 6] used to hold the capsule in place and capsule surface roughness, as well as low mode asymmetries. High adiabat implosions, designed to reduce the effect of ablation front hydrodynamic instabilities, as well as a reduced convergence, are impacted primarily by low mode implosion shape. In both cases, shape and hydrodynamic instability play a key role. While there may be other degradation mechanisms, they are difficult to assess without addressing the afore-mentioned issues. Thus, the near term goal for ICF experiments on the NIF is to mitigate these known effects by pushing towards round implosions while addressing high mode perturbations to maximize one-dimensional (1D) implosion behavior, i.e., high yield over 1D clean (no mix) simulations. The most straight forward means to do this is to retreat from aggressive ICF designs by reducing the implosion convergence, defined as the initial outer radius to the final hot spot x-ray self-emission radius, and increasing the case-to-capsule ratio, defined as the hohlraum to capsule outer radius. This should provide a more predictable platform that can be used to study ignition physics, test mitigation strategies for uncovered issues, benchmark simulations models, and ultimately to evaluate what is needed to achieve ignition.
Los Alamos National Laboratory (LANL) has adopted the strategy of increasing the case-to-capsule ratio and reducing the convergence ratio for each of its ignition campaigns as a means to achieve 1D like behavior. There are currently three major ICF campaigns under way at LANL. The first is a large case-to-capsule, low radiation temperature target design that takes advantage of beryllium capsules[7–10]. The second approach uses Deuterium–Tritium (DT) liquid layered targets in which the convergence ratio is controlled by varying the initial mass in the central cavity of the capsule through the vapor pressure. The last approach uses double shell targets that are comprised of an inner shell filled with the DT fuel surrounded by an outer shell with a foam fill between the shells. The inner shell filled with the fusion fuel is expected to have a convergence of
2 High case-to-capsule ratio, low radiation temperature beryllium capsule campaign
One of the goals of the Los Alamos program is evaluate the benefits of beryllium capsules compared to other ablator options such as plastic (CH) or high density carbon (HDC). During the first beryllium ablator campaign, the target design for the campaign took advantage of the hohlraum development by the high foot campaign[1–3, 11] to minimize the number of shots needed to optimize the drive and provide the ability for some cross capsule comparisons. However, the target was not optimized for beryllium capsules. The experiments appear to show that beryllium capsules do not significantly change the hohlraum conditions compared to the CH ablator[12]. However, the implosions for both ablators were plagued by the same poor symmetry control making it difficult to compare based on nuclear performance. This is consistent with the work of Clark
Fig. 1. Plot of radiation flux symmetry versus case-to-capsule ratio for Legendre modes P2 and P4 normalized by the flux on the capsule.
While it is known that increasing the case-to-capsule ratio (ratio of hohlraum to capsule radius) smooths the x-ray radiation pattern on the capsule, the benefits of radiation smoothing is not a strong function above a case-to-capsule ratio of
Fig. 2. Plots of the hohlraum cross sections showing the laser ray traces with the hohlraum density for a (a) 2200 and (b) $1290~\unicode[STIX]{x03BC}\text{m}$ outer diameter beryllium capsule. The yellow regions correspond to densities greater the $1/4$ critical for 351 nm light.
With this in mind, a series of 1D clean simulations, i.e., no mix, have been completed to evaluate the energetics in case-to-capsule ratio versus convergence ratio space as shown in Figure
Fig. 3. Case-to-capsule ratio versus convergence ratio design space for 1D simulations for beryllium with both two and three shock pulse shapes. Plot includes the design point for wetted foam targets and typical ignition targets. The size of the points is proportional to neutron yield.
For the upcoming high case-to-capsule ratio campaign, an
Fig. 4. (a) Pie diagram for the $1600~\unicode[STIX]{x03BC}\text{m}$ diameter capsule using a three shock pulse shape. (b) Radiation drive history for both the two and the three shock design using a 6.72 mm diameter hohlraum. (c) Laser power histories driving the two drives.
Given the conservative target design, we expect to produce symmetric implosions with little time dependent swings. If we do achieve a 1D like implosion, the next step will be to move towards larger capsules to determine at which case-to-capsule ratio symmetry control for beryllium capsules degrades. From those experiments, we can hydro-scale the target and get an estimate of how much energy would be needed to achieve ignition with the low radiation temperature, high case-to-capsule beryllium capsule target design.
3 Liquid layers
Liquid fuel layers are an alternative approach to control the convergence ratio[17, 18]. Unlike DT ice layers which must be fielded below the triple point for the DT fuel, liquid layers can be fielded over a range of temperatures from
Recent technological advances in target fabrication techniques for producing foam lined HDC capsules has made liquid layer implosions possible[19, 20]. Unlike previous efforts in which CH was coated onto a foam sphere, generating the foam inside an HDC capsule produces a smooth interface between the foam and the ablator. In addition, the use of HDC capsules provides opportunities for a wider range of target designs such as vacuum hohlraums, and for potentially better ablation front stability control through reduced surface roughness[21, 22]. While a sufficient number of capsules are currently being produced for experiments, there are still technical challenges for mass production of capsules with consistent foam characteristics such as thickness and uniformity. This needs to be addressed before transitioning from a research to a production project. Another technical issue being addressed is the size of the fill tube. Currently, a
Fig. 5. Pie diagrams for (a) liquid layer and (b) ice layered[22] targets using HDC capsules.
Recently, the first liquid layered target experiments were successfully fielded on NIF demonstrating the capability[23]. The target used a near vacuum hohlraum design with a
Fig. 6. (a) X-ray image through the laser entrance hole of first liquid layered target fielded on NIF using liquid $D_{2}$ . (b) Unwrapped image.
Fig. 7. (a) Example of a double shell target. (b) Example laser pulse shape for an indirect double shell design. (c) Example of a double shell implosion in Lagrangian coordinates showing the collisions of the two shells and compression of the inner shell.
4 Double shells
Double shell targets provide a very different approach to ICF than single shells[24–30]. Double shell targets consist of a low
Unlike single shells that use a central hot spot to ignite and initiate burn of the cold dense fuel surrounding the hot spot, double shells ignite via volume ignition, i.e., heating the entire fuel volume to fusion conditions. Volume ignition with similar laser energies as for hot spot ignition is possible because of several design factors. The inner shell is made of a mid- to high-
Double shell targets have a different set of concerns than single shell targets. For double shells, the ablative physics of the outer shell, the inflight aspect ratio, pulse shaping and the fact that a fuel layer is not needed are advantages. In addition, the implosions do not require high implosion velocities. The primary trade-off is the engineering challenge of building the precision double shell targets. For double shells the other key challenges compared to single shell hot spot ignition is mix at the inner shell fuel interface and preheating of the inner shell by high energy x-rays. Mixing of high
5 Conclusions
As we continue to march towards ICF ignition, many challenges remain. Experiments have identified several issues which are in the midst of being addressed. To assist our understanding in order to mitigate these problems, to identify other potential problems, and to evaluate modeling deficiencies, it is prudent to develop target implosions that behave as near as possible to 1D simulations, i.e., ideal as possible. This means developing less stressful target designs by increasing the case-to-capsule ratio and reducing the implosion convergence. LANL has adopted three approaches in line with this strategy, a low radiation temperature beryllium target design, liquid fuel layer targets, and double shell targets. These platforms are being developed to support ignition science experiments needed to provide guidance on the most likely path towards ignition.
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Article Outline
J. L. Kline, S. A. Yi, A. N. Simakov, R. E. Olson, D. C. Wilson, G. A. Kyrala, T. S. Perry, S. H. Batha, E. L. Dewald, J. E. Ralph, D. J. Strozzi, A. G. MacPhee, D. A. Callahan, D. Hinkel, O. A. Hurricane, R. J. Leeper, A. B. Zylstra, R. R. Peterson, B. M. Haines, L. Yin, P. A. Bradley, R. C. Shah, T. Braun, J. Biener, B. J. Kozioziemski, J. D. Sater, M. M. Biener, A. V. Hamza, A. Nikroo, L. F. Berzak Hopkins, D. Ho, S. LePape, N. B. Meezan, D. S. Montgomery, W. S. Daughton, E. C. Merritt, T. Cardenas, E. S. Dodd. Developing one-dimensional implosions for inertial confinement fusion science[J]. High Power Laser Science and Engineering, 2016, 4(4): 04000e44.